Deployments of IMS (IP Multimedia Subsystem) based communication networks are progressing worldwide. In a typical deployment scenario an IMS network is built up as a parallel network to an existing PSTN (Public Switched Telephone Network), wherein the IMS network and the PSTN are usually operated by different network operators, or by different divisions of a large operator.
In large countries the geographical area of the country is divided into regions. This division is usually driven by the density of the population. Communication networks follow this sub-structuring by offering communication services in the different regions, wherein the different regions may be served by different network operators.
In order to allow people to communicate freely with each other although belonging to different network operators, the IMS network and the PSTN network are interconnected via so called POIs (Point of Interconnect). At these POIs telephone calls are routed between the peering networks. The routing of a telephone call from the own network to a peering network is also known as breakout from the own network.
This scenario is depicted in more detail in FIG. 1. Region 1 100 and region 2 102 are both covered by the IMS based communication network 101 and by the PSTN 112. The region 1 100 and region 2 102 of the IMS network 101 are using a common IP Backbone transport network 110.
The IMS network 101 and the PSTN 112 are interconnected at the POI 114 in region 100 and at POI 120 at region 102.
The IMS network 101 comprises, among other nodes, the IMS infrastructure nodes MGCF 116 (Media Gateway Control Function) handling POI 114 in region 1 100, MGCF 122 handling POI 120 in region 102, a S-CSCF 124 (Serving Call Session Control Function), a TAS 126 (Telephony Application Server) implementing the subscriber services, and a BGCF 128 (Breakout Control Function) controlling the breakout of sessions from the IMS network 101 to the PSTN 112. The S-CSCF 124, TAS 126, and the BGCF 128 are located in region 1 100, while in the region 2 102 none of such nodes (124, 126, 128) are located. This is a typical deployment where the main IMS infrastructure nodes are located centrally in very few regions only.
Now looking closer at a simplified view of routing of a session invitation, a UE (user equipment) A 104 in region 1 100 initiates a session invitation to a UE B 106 in region 2 102. The routing path is illustrated by the solid line 150. The session invitation is routed to the S-CSCF 124 allocated to the B-subscriber using UE B 106, which then involves the TAS 126 for handling the terminating services for the B-subscriber. Then the session is delivered to the UE B in region 2 102. Since the session is routed using the transport IP backbone 110 of the IMS network 101, the handling of the session is very efficient as the entire session can be handled within the IMS network 101.
Now the B-subscriber using UE B 106 activates a forwarding service, for example unconditional session forwarding to a UE C 108 of the PSTN 112. The forwarding service is executed in the TAS 126, and the resulting routing path is illustrated by the dashed line 160. The S-CSCF 124 determines the breakout condition and routes the session invitation to the BGCF 128 for breakout handling. Here it is assumed that the B-subscriber does not have a TEL IMPU (Telephony IP Multimedia Public Identity), but only a SIP IMPU (Session Initiation Protocol IP Multimedia Public Identity) with a common domain part for all subscribers in region 1 100 and region 2 102, for example subscribername@sip.IMSoperator.com. Here “IMSoperator” would correspond to the common domain part. So the common domain part would not reveal in which region the UE B 106 is located.
Since the BGCF 128 cannot derive where the UE B 106 or the UE C 108 are located, it routes the forwarded session invitation to the closest POI to the PSTN 112, which is the POI 114 in the region 1 100. So the BGCF 128 sends the session invitation to the MGCF 116 handling POI 114. The MGCF 116 then converts the session invitation to a corresponding telephone call setup request compatible with the PSTN 112. Then the PSTN 112 takes care of the routing of the telephone call setup request to the UE C 108 in region 2 102.
As can be derived from the above routing example, the handling of the session is not efficient as the routing of the corresponding telephony call from region 1 100 to region 2 102 is done by the PSTN 112, which involves usage of expensive PSTN resources. In this case the IMS network 101 operator has to pay a high termination fee to the PSTN 112 operator. The termination fee is typically proportional to the routing distance in the network until the termination point is reached, which in this case is the PSTN 112 UE C 108. There is clearly a need to improve the routing of communication sessions from the IMS network 101 to the PSTN 112, or in general to other communication networks, in terms of efficiency.